1. Field of the Invention
This invention relates generally to an apparatus and method of making improved superabrasive tools, such as a centerless grinding wheel, for example.
2. Description of the Related Art
Many types of abrasive tools, such as centerless grinding wheels for example, have been manufactured using a compression molding process to form the outer abrasive surface of the wheel. Large presses involving heat and high pressures, such as 1,000 to 10,000 psi, are employed to compact a powdered composition comprising a resin, metal or ceramic binder, filler materials and superabrasive particles, such as diamonds, for one example, in a mold.
The largest number of such abrasive grinding wheels comprise a resin bonding composition with a variety of such compositions well-known to those skilled in the art and designed for particular applications or as a matter of designer's choice.
One of the problems of the prior conventional molding apparatus and process of this type is the limitation of the axial length dimension which may be acceptably made. This limits the effective width of the annular configuration formed. Typically, only a relatively narrow wheel configuration having an axial length dimension of no more than about 1 to 2 inches may be formed depending upon the particular components of the abrasive composition used and the thickness of the layer of abrasive molding composition formed. Many industrial applications require a grinding surface having up to about 24 inches of width or axial length. Using conventional methods, this requires the separate molding of a plurality of 1 to 2 inch long annular or tubular forms and then adhesively stacking one upon the other to obtain the required axial length. Clearly, this is a costly, labor intensive effort which results in a grinding wheel with seams formed between each of the stacked narrow annular components. Further, since each annular component is made during a separate molding process, uniformity of the properties of each component may vary more than desired.
Since the conventional compression molding apparatus uses annular mold spaces and axially moveable plungers having inflexible surfaces to compact the abrasive composition, the ability to compress the abrasive filled molding composition in an acceptable uniform manner is limited to these narrow axial dimensions. Prior attempts to compress greater depths were unsuccessful as the range of pressure applied throughout the molding composition varied too much to achieve a sufficient uniformity of density and surface hardness for practical industrially acceptable products. Further, axial depths greater than about 2 or 3 inches would require compression molding machines which apply excessively greater pressure and at some point are impractical for this use.
It should be noted that the abrasive molding compositions used typically are a mixture of relatively fine powder size components. The object of the process is to reduce the porosity of the final compacted and molded product to as close to zero as feasible. However, the fine powder component mixture represents solids during the initial compaction. The binder component eventually becomes somewhat viscous or semi-solid during the heating and pressing cycle in order to wet and bond the remaining solid particles in a strong, dense final product.
The filler and abrasive particles have limited flow properties within the mold cavity even under high pressure. Therefore, in the conventional axially directed pressing method, the axial length of the composition in the mold tends to be limited to between the 1 to 2 inches noted to obtain a practical industrially accepted final product. The use of press platens having inflexible surfaces tends to limit the uniformity of density and surface hardness achieved even in non-annular shapes such as in laps and similar abrasive tools.
It should be noted that once the annular abrasive molding composition is processed, the interior volume is filled with a suitable core material. Often the core is a plastic material which is bonded to the inner surface of the abrasive layer ring-shaped configuration and completes the grinding wheel tool.
Prior to the present invention, significant improvement of compression molding of annular shapes of abrasive products, such as described, has eluded those skilled in the art.